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Friday, 3 February 2017

Neutral Control and Coordination II


Generation and Conduction of Nerve Impulse
  • Ions channels are present in neural membrane which is selectively permeable to different ions. When neuron is not conducting impulse (resting), axonal membrane is more permeable to K+ ions and impermeable to Na+ ions.
  • Ionic gradient across the resting membrane is maintained by active transport of ions by sodium-potassium pump. This will develop positive charge outside the axonal membrane and negative charge on inner side.
  • The electrical potential difference across the resting membrane is called resting potential.
  • When stimulus is applied at site A, the membrane becomes permeable to Na+ ions to make rapid influx of Na+ ions to create outer surface negatively charged and inner membrane positively charged that create nerve impulse.
  • The nerve impulse from A moves to B in inner surface and B to A on outer surface. This process is repeated several times to transmit the impulse.
Nerve impulse is transmitted from one neuron to another neuron through synapse.
There are two types of synapse-
a. Electrical synapse - the membrane of pre and post synaptic neuron is very close to each other and current flow directly from one neuron to another.
b. Chemical synapse – pre and post synaptic neuron is separated by fluid filled space called synaptic cleft. Neurotransmitter are involved in transmission of impulses.

Central Neural System - Brain is the central information processing of our body and act as command and control centre. Human brain is protected by skull (cranium) and cranial meninges, three layered membrane, outer dura meter, middle arachnoid and inner pia meter.
Brain can be divided 3 parts- forebrain, midbrain and hindbrain.

Forebrain- consists of cerebrum, thalamus and hypothalamus. Cerebrum is divided into left and right cerebral hemispheres which are covered by cerebral cortex (grey matter). Cerebral cortex contains sensory neuron, motor neuron and association area. Association area controls the memory and communication like complex process. Inner part of cerebral hemisphere forms the white matter that control sensory and motor signaling.
Hypothalamus controls the urge for eating, drinking and body temperature. They also release hypothalamic hormones. Limbic system is involved in controlling sexual behavior and expression of emotional reactions.

Midbrain is located between hypothalamus and pons of hindbrain. Dorsal portion consists of four round lobes called corpora quadrigemina. They are involved in relay of impulses back and forth between cerebrum, cerebellum, pons and medulla.

Hind brain consists of pons, medulla oblongata and cerebellum. Pneumatic centre is present in hindbrain that control inspiration. They also relay impulses between the medulla and superior part of brain. Cerebellum controls balance and posture.

Reflex action is a spontaneous autonomic mechanical response to a stimulus without the will of the organism. It is controlled by spinal cord. The afferent neuron receives the signal from sensory organs and transmits the impulse to CNS. The efferent neuron carries the impulse from CNS to effector. Ex- knee-jerk reflex. The path followed by reflex action is called reflex arc.

Human Eye – spherical structure consists of three layers, external layer is sclera whose inner most layers is called cornea, middle layer choroid and innermost layer is called retina. The light rays of visible wavelength fall on retina through cornea and lens to generate impulses in rods and cones. Photosensitive pigments opsin and retinal get dissociated due to light to change its shape. Change in shape of opsin cause change of permeability to generate action potential that is transmitted to brain via optic nerve.
Human Ears
Divided into three regions outer ear, middle ear and inner ear. External ear receives the sound wave and directs them to ear drum. Vibration of ear drum leads to vibrate ear bones. The vibration reaches to cochlea that generate wave in lymph. The waves generate ripples in basilar membrane and hair in them. Nerve impulse are generated in afferent neuron that passes to brain via auditory nerves.
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